This invention relates to robotic mechanisms that manipulate media cartridges in an automated media cartridge storage library subsystem, and in particular, to a fully redundant system that includes a plurality of independent robots in the form of robotic pods. The robotic pods are concurrently operational on a vertical wall of media cartridge storage cells and media cartridge players to both transport selected media cartridges among the media cartridge storage cells and media cartridge players and perform other library system tasks.
It is a problem in automated media cartridge storage library subsystems to serve the media cartridge mount requests that are received from an associated host processor in a timely and efficient manner. Existing automated media cartridge storage library subsystems (library systems) are capable of storing and retrieving large quantities of information that are stored on media cartridges. This is accomplished by the use of a large number of cartridge storage cells, each of which houses a single media cartridge, that are housed within an enclosure. Such library systems utilize a robotic mechanism, such as a robotic arm, to quickly move the media cartridges between their media cartridge storage cells and media cartridge players. For example, to retrieve information that is stored on a selected media cartridge, the robotic arm is moved to a location opposite the media cartridge storage cell that houses the selected media cartridge. An end effector of the robotic arm then grasps the media cartridge and extracts it from the media cartridge storage cell. The robotic arm then repositions itself from the media cartridge storage cell to a media player, where the end effector loads the media cartridge into the media player.
The problem with this architecture is that the robotic arm must wait for the previously mounted media cartridge to be dismounted and put away. The media cartridge mount cycle requires exclusive use of the robotic arm, even though another requested mount cycle may be waiting in a queue for the robotic arm to become available. A reduction in usable duty cycle is also incurred if the robotic arm is requested to move media cartridges from a library system loading window to their assigned media cartridge storage cells or the robotic arm passes media cartridges between library enclosures.
A further problem with existing library systems is that a single robotic arm system requires expensive mechanics to maintain reliability levels common to the high duty cycle of the industry, and an inoperable robotic arm shuts down the complete system for repair. Single arm library systems, as typified in U.S. Pat. No. 5,143,193 have been improved to provide redundant robotic mechanisms in order to mitigate the effects of these problems. For example, a double end effector is disclosed in U.S. Pat. No. 4,907,889 to thereby allow a single robotic arm to simultaneously carry two media cartridges or to survive the loss on one of the end effectors. The library design of U.S. Pat. No. 5,456,569 has two independent end effectors and two vertical moveable means to provide even more redundancy. The two robot system of U.S. Pat. No. 4,937,690 shows how two complete robotic arms can overlap in a common robotic space. A method with multiple accessors is disclosed in the design of U.S. Pat. No. 5,395,199 whereby each accessor is semi-independent of the others to move multiple media cartridges at once. These designs show improved methods of incorporating multiple robotic arms and/or multiple segments of the robotic arms into a given library space.
A further problem is that library system activities are constantly interrupted by overhead tasks. The storage library contents are typically monitored with label readers incorporated on the robotic arm, and the service of the robotic arm is required in such cases where label reading is required. Also, robotic arm positioning within the robotic space is usually accomplished with an optical or other locating device that is mounted on the robotic arm, requiring that the robotic arm be used solely to calibrate the position of the end effector with respect to the media cartridges. Label reading and calibration are time consuming tasks which impact the duty cycle of a library system. Robotic arm calibration is necessary to ensure positional accuracy, and a large robotic arm may have extensive calibration devices on board the robotic arm, adding to the cost and weight of the robotic arm. The typical robotic arm and its supporting structure requires several servo motors to move the robotic arm between positions. Each move of the robotic arm requires a time interval after the mechanism has stopped to bring the servo position into a steady state. The servo mechanism settling time depends on the stiffness of the robotic arm and can represent a substantial portion of the total robotic duty cycle. Large robotic arms with poor stiffness have longer settling times than small stiff robotic arms. The moving mass of the robotic arm is much greater than the media cartridge being moved. It is common practice to reduce mass through the use of high-technology materials, although technological advances in materials do not give an adequate cost/performance solution to this problem. The moving mass of the robotic arm also relates directly to power consumption, which is an important factor in large installations. A large storage library may take up a considerable amount of floor space, primarily due to the robotic arm design. The typical design utilizes a large swept space for the robotic arm, while the media volumetric space is only a fraction of the total space occupied by the library system. The larger the robotic arm, the more volume taken as the robotic arm is moved about during operation. A large space is also used if the media cartridges are stored in a horizontal plane rather than a vertical plane. Vertically oriented storage walls of media cartridges are preferred over horizontal storage due to the fact that floor space is at a premium while room space is often available in the vertical direction. The system of U.S. Pat. No. 5,395,199 shows a horizontal storage array that covers a computer room floor, but does not make use of the rest of the space above the floor. In addition, vertical media storage planes accommodate an operator in between media cartridge storage walls during maintenance, without compromising operation of a multi-library system.
The above described problems are solved and a technical advance achieved by the present automated library system that contains multiple independent robots for concurrently manipulating multiple media cartridges. The library system comprises a two-dimensional array that contains media cartridge storage cells and media cartridge players. A system of rails is used to guide robotic pods through all of the locations in the array, which eliminates the need for any steering or guide mechanism on board the robotic pods, resulting in a reduction in the mass of the robotic pods. The rail system also constrains the movement of the robotic pods into horizontal and vertical movements, thereby simplifying the control algorithms for collision avoidance that are required by a typical random moveable object handling system based on horizontal, vertical and diagonal degrees of freedom. The robotic pods contain a moveable carriage that is capable of transporting robotic components, such as media cartridge pickers, bar code reading devices, and other task oriented sub-modules, in the storage library rail system. For example, the robotic pod carriage assemblies can be fitted with maintenance devices, monitoring instruments or cleaning apparatus to give added flexibility to the rail system architecture.
The rail system is implemented with a rack and pinion type drive gear interface, rather than a rubber tire type friction drive, to thereby allow for improved traction and increased operating speeds of the robotic pods. Such a drive system can be integrated with position sensing encoders that are located on each robotic pod drive motor to provide precise locating of the robotic pod on a wall of the array. Typical robotic arms require the continuous execution of calibration algorithms to achieve the necessary positioning accuracy, using optical or mechanical devices in the process. The robotic pods do not require continuous calibration because the relationship between media cartridge storage cells and the robotic pods is made much more precise by the close proximity of the robotic pod to the wall structure and the use of the position encoders on each robotic pod. The rail system also allows for the incremental expansion of the library system by simply adding more rail, and the library system can also be simply partitioned into separate modules. Furthermore, media cartridges can be shared between libraries by adding pass through rails that interconnect the libraries.
In this library system, the moving mass and volume of a robotic pod is substantially less than a comparable robotic arm type device. The reduction in mass of the robotic mechanism allows for higher operating speeds, which results in improved performance of the library system, at lower power consumption. Typical robotic servo settling times are also reduced because of the elimination of large mechanical elements. Reduction in robotic mass further allows large library systems to be placed on lighter weight computer floors, or even in non-computer environments such as ships, light weight office structures and residential buildings.
In addition, the multiple robot architecture reduces the need for very high reliability and costly robotic mechanisms. This is because the media cartridge manipulation work that a robotic arm does in a typical library system can now be divided among multiple robotic pods. Each robotic pod performs only a fraction of the media mount operations and failure resiliency is an added benefit to the multiple redundant robot architecture. A service technician can easily retrieve a dysfunctional robotic pod from a library system enclosure without disabling the entire library system. Manufacturing of the multiple independent robots involves fewer parts and less overhead due to decreased floor space, easier testability and faster build cycles. Engineering changes at the factory and in the field involve only the individual robotic pods and rails, where an arm type robot is subject to downtime and increased repair cost because of the complexity of a large assembly. Upgrading old assemblies or adding new robotic pods to an existing library involves simply opening a service port in the library system enclosure, where robotic pods can be loaded or unloaded from the rail system. Library system performance enhancements can be realized and cost levels established by adding and subtracting robotic pods.